Enhanced densification and ionic conductivity of Li-garnet electrolyte: Efficient Li2CO3 elimination and fast grain-boundary transport construction

As promising next generation energy storage system, all solid state lithium batteries offer great potential for achieving high energy density and improved safety over lithium batteries with liquid counterpart. Dense microstructure with high ionic conductivity are prerequisites for solid electrolyte. The promising Li-garnet electrolyte suffers from poor sinterability, which results in porous structure and large grain boundary resistance. Herein, we report that the poor sinterability of garnet Li7La3Zr2O12 is most likely related to the Li2CO3 formed on particle surface, and SiO2 is an efficient additive to scavenge the surface Li2CO3 and so significantly improve the densification of electrolyte. The reaction product Li4SiO4 of SiO2 with Li2CO3 is a Li-ion conductor, which exists at grain boundary and so can facilitate the grain boundary lithium ion diffusion. An addition of 1 mol.% SiO2, with respect to the total lithium content in Li6.4La3Zr1.4Ta0.6O12 (LLZTO), significantly increases the relative density of LLZTO pellet by 28% and the conductivity by one order of magnitude, reaching 3.84 x 10(-4) S cm(-1) at room temperature by a conventional solid state sintering. A symmetric lithium cell assembled with such dense LLZTO electrolyte can cycle well for 260 h compared with unmodified LLZTO running only for 72 h before shortcircuit.